Semiconductor device

ABSTRACT

At least one of a plurality of transistors which are highly integrated in an element is provided with a back gate without increasing the number of manufacturing steps. In an element including a plurality of transistors which are longitudinally stacked, at least a transistor in an upper portion includes a metal oxide having semiconductor characteristics, a same layer as a gate electrode of a transistor in a lower portion is provided to overlap with a channel formation region of the transistor in an upper portion, and part of the same layer as the gate electrode functions as a back gate of the transistor in an upper portion. The transistor in a lower portion which is covered with an insulating layer is subjected to planarization treatment, whereby the gate electrode is exposed and connected to a layer functioning as source and drain electrodes of the transistor in an upper portion.

TECHNICAL FIELD

An embodiment of the present invention relates to a semiconductordevice, particularly a semiconductor device including a memory elementand an inversion element.

BACKGROUND ART

In recent years, metal oxides having semiconductor characteristics(hereinafter, referred to as oxide semiconductors) have attractedattention. The metal oxides having semiconductor characteristics can beapplied to transistors (Patent Document 1 and Patent Document 2).

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

An object of one embodiment of the present invention is to control thethreshold voltage of at least one of a plurality of transistors whichare highly integrated in an element. Further, an object of oneembodiment of the present invention is to provide a structure whichenables control of the threshold voltage of a transistor withoutcomplicating a manufacturing process.

One embodiment of the present invention is an element in which aplurality of transistors are longitudinally stacked. At least atransistor in an upper portion includes metal oxide having semiconductorcharacteristics. Part of the same layer as a gate electrode of atransistor in a lower portion is provided to overlap with a channelformation region of the transistor in an upper portion, so that the partof the same layer as the gate electrode functions as a back gate of thetransistor in an upper portion.

Here, the transistor in a lower portion is subjected to planarizationtreatment on the condition of being covered with an insulating layer,whereby the gate electrode of the transistor in a lower portion isexposed and connected to a layer functioning as a source or drainelectrode of the transistor in an upper portion.

Note that the part functioning as the back gate is not overlapped with asemiconductor layer in a lower portion; thus the insulating layer isleft over the part functioning as the back gate, and the part of thesame layer as the gate electrode of the transistor in a lower portionand a semiconductor layer of the transistor in an upper portion overlapwith each other with the insulating layer interposed therebetween.

The threshold voltage of at least one of a plurality of transistorswhich are highly integrated in an element can be controlled.Furthermore, such a control of the threshold voltage of the transistorcan be achieved without complicating the manufacturing process.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C illustrate a memory element according to Embodiment 1.

FIG. 2 is a diagram of a memory device including a memory elementaccording to Embodiment 1.

FIG. 3 is a timing chart showing operation of the memory device of FIG.2.

FIGS. 4A and 4B are diagrams each showing a read-out circuit in a memorydevice according to Embodiment 1.

FIGS. 5A to 5H illustrate a manufacturing method of a memory elementaccording to Embodiment 1.

FIGS. 6A to 6G illustrate a manufacturing method of a memory elementaccording to Embodiment 1.

FIGS. 7A to 7D illustrate a manufacturing method of a memory elementaccording to Embodiment 1.

FIGS. 8A to 8C illustrate a memory element according to Embodiment 2.

FIGS. 9A to 9C illustrate an inversion element according to Embodiment3.

FIGS. 10A to 10C illustrate an inversion element according to Embodiment4.

FIGS. 11A to 11C illustrate a memory element according to Embodiment 5.

FIGS. 12A to 12C illustrate a memory element according to Embodiment 6.

FIGS. 13A to 13C illustrate a memory element according to Embodiment 7.

FIGS. 14A to 14C illustrate a memory element according to Embodiment 8.

FIGS. 15A to 15F illustrate electronic devices according to Embodiment9.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail withreference to drawings. However, the present invention is not limited tothe description below, and it is easily understood by those skilled inthe art that modes and details disclosed herein can be modified invarious ways without departing from the spirit and the scope of thepresent invention. Therefore, the present invention is not construed asbeing limited to description of the embodiments.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention will be described. As the semiconductor device, amemory device is specifically described in this embodiment.

FIG. 1A is an example of a circuit diagram of a memory element includedin the memory device of this embodiment.

The memory element illustrated in FIG. 1A includes a transistor 100, atransistor 102, and a capacitor 104. In FIG. 1A, one of source and drainelectrodes of the transistor 100 is electrically connected to a firstwiring 111, and the other of the source and drain electrodes of thetransistor 100 is electrically connected to a second wiring 112. One ofsource and drain electrodes of the transistor 102 is electricallyconnected to a third wiring 113, and a gate electrode of the transistor102 is electrically connected to a fourth wiring 114. Then, a gateelectrode of the transistor 100 and the other of the source and drainelectrodes of the transistor 102 are electrically connected to one ofelectrodes of the capacitor 104. The other electrode of the capacitor104 is electrically connected to a fifth wiring 115. The transistor 102is further provided with a back gate BG functioning as another gateelectrode.

Here, a transistor including an oxide semiconductor in a channelformation region is employed as the transistor 102. The transistorincluding an oxide semiconductor is highly purified by removal ofhydrogen and water, whereby the off-state current can be significantlydecreased. Therefore, electric charges given to the gate electrode ofthe transistor 100 can be held for an extremely long time by turning offthe transistor 102. Further, provision of the capacitor 104 facilitatesholding of electric charges given to the gate electrode of thetransistor 100 and reading out of held data.

Operations of writing data, holding data, and reading out data in thememory element illustrated in FIG. 1A are described below.

First, the transistor 102 is turned on by supplying potential of thefourth wiring 114, and then electric charges supplied from the thirdwiring 113 are supplied to the gate electrode of the transistor 100 andthe one of the electrodes of the capacitor 104. In other words, electriccharges are supplied to a floating gate portion (FG portion in FIG. 1A)where the other of the source and drain electrodes of the transistor102, the one of the electrodes of the capacitor 104, and the gateelectrode of the transistor 100 are electrically connected (writingoperation). Either of two types of electric charges having differentpotential levels is supplied here. Electric charges having low potentiallevel are referred to as “low-level electric charge”, and electriccharges having high potential level are referred to as “high-levelelectric charge”.

After that, the transistor 102 is turned off by supplying potential ofthe fourth wiring 114, so that electric charges at the FG portion ofFIG. 1A are held (holding operation). The off-state current of thetransistor 102 can be significantly decreased; thus, the electriccharges stored in the FG portion can be held for a long time.

Next, reading of data will be described. By supplying an appropriatepotential (reading potential) to the fifth wiring 115 while apredetermined potential (constant potential) is supplied to the firstwiring 111, the potential of the second wiring 112 varies depending onthe amount of electric charges held in the FG portion (potential of thegate electrode of the transistor 100). This is because in general, whenthe transistor 100 is an n-channel transistor, an “apparent thresholdvoltage V_(th) _(_) _(H)” in the case where a high-level electric chargeis given to the gate electrode of the transistor 100 is lower than an“apparent threshold voltage V_(th) _(_) _(L)” in the case where alow-level electric charge is given to the gate electrode of thetransistor 100. Here, an “apparent threshold voltage” refers to thepotential of the fifth wiring 115, which is needed to turn on thetransistor 100 when the first wiring 111 has a constant potential. Thus,when the potential of the fifth wiring 115 is set to a potential V₀intermediate between V_(th) _(_) _(H) and V_(th) _(_) _(L), electriccharges given to the gate electrode of the transistor 100 can bedetermined. For example, in the case where a high-level electric chargeis given, when the potential of the fifth wiring 115 is set to V₀(>V_(th) _(_) _(H)), the transistor 100 is turned on. In the case wherea low-level electric charge is given, when the potential of the fifthwiring 115 is set to V₀ (<V_(th) _(_) _(L)), the transistor 100 remainsin an off state. Therefore, the held data can be judged and read outwith reference to the potential of the second wiring 112.

Note that in the case where memory elements are arranged in matrix, dataof only the desired memory element is read out. In order to read data ofonly the desired memory element and not to read data of the other memoryelements, in the case where the transistors 100 are connected inparallel among the memory elements, a potential which allows thetransistors 100 to be turned off (potential lower than V_(th) _(_) _(H))regardless of a state of the gate electrode may be supplied to the fifthwirings 115 in the memory elements whose data are not to be read. On theother hand, in the case where the transistors 100 are connected inseries among the memory elements, a potential which allows thetransistors 100 to be turned on (potential higher than V_(th) _(_) _(L))regardless of a state of the gate electrode may be supplied to the fifthwirings 115 in the memory elements whose data are not to be read.

Next, rewriting of data is described. Rewriting of data is performed ina manner similar to that of the writing and holding of data. That is,the transistor 102 is turned on by potential of the fourth wiring 114.Thus, the potential of the third wiring 113 (a potential related to newdata) is supplied to the FG portion. After that, the transistor 102 isturned off by potential of the fourth wiring 114; thus, electric chargewith potential level related to new data is given to the FG portion.

In the memory element illustrated in FIG. 1A, data can be directlyrewritten by overwriting data as described above. For that reason, highvoltage with which electric charge is extracted from a floating gate ina flash memory or the like is not necessary, and a decrease in operatingspeed due to injection of an electric charge to a floating gate andremoval of an electric charge from a floating gate can be suppressed.

Note that the other of the source and drain electrodes of the transistor102 and the gate electrode of the transistor 100 are electricallyconnected, whereby the FG portion in FIG. 1A has a function equivalentto that of a floating gate of a flash memory. When the transistor 102 isoff, the FG portion can be regarded as being embedded in an insulatorand electric charges can be stored in the FG portion. The transistor 102provided in the memory element illustrated in FIG. 1A includes a channelformation region formed using an oxide semiconductor, and the off-statecurrent of the transistor 102 can be about 100000 times as low as thatof the conventional transistor 102 including silicon or the like. Thus,it can be assumed that leakage of electric charges from the FG portionthrough the transistor 102 hardly occurs. Therefore, with use of thememory element illustrated in FIG. 1A, a nonvolatile memory device whichcan hold data even without supply of power can be provided.

For example, when the off-state current of the transistor 102 is 10zA/μm or less at room temperature and the capacitance value of thecapacitor 104 is approximately 10 fF, data can be held for at least 10⁴seconds or longer. Note that this data holding time depends oncharacteristics of the transistor 102 and the capacitance value of thecapacitor 104.

Further, in the memory element illustrated in FIG. 1A, tunneling currentdoes not flow in an insulating layer between the channel formationregion and the FG portion and thus, the insulating layer does notdeteriorate, which differs from a flash memory. Therefore, there is nolimitation on the number of writing operations. Furthermore, a highvoltage needed for writing or erasing in a conventional floating gatetransistor is not necessary.

When the gate leakage of the transistor 102 is sufficiently low, anelectric charge holding period (also referred to as a data holdingperiod) is determined depending on the off-state current of thetransistor 102 mainly, in such a condition that R₁ is higher than R_(OS)and R₂ is higher than R_(OS), where R_(OS) indicates the resistancevalue (also referred to as effective resistance) between the sourceelectrode and the drain electrode of when the transistor 102 is off, R₁indicates the resistance value of an insulating layer included in thecapacitor 104, and R₂ indicates the resistance value of the gateinsulating layer of the transistor 100.

On the other hand, when the conditions are not satisfied, it isdifficult to sufficiently secure the holding period even if theoff-state current of the transistor 102 is decrease enough. This isbecause a leakage current other than the off-state current of thetransistor 102 (e.g., a leakage current generated between the sourceelectrode and the gate electrode) is large. Thus, in the memory elementillustrated in FIGS. 1A to 1C, the preferable resistance relation isthat R₁ is higher than R_(OS) and R₂ is higher than R_(OS).

Further, the capacitance value C₁ of the capacitor 104 is equal to orhigher than the capacitance value C₂ of the transistor 100. When C₁ ishigher, variation in the potential of the fifth wiring 115 can besuppressed when the potential of the FG portion is controlled by thefifth wiring 115 (e.g., at the time of reading).

Note that the resistance values R₁ and R₂ and the capacitance values C₁and C₂ are determined depending on materials and the thicknesses of thegate insulating layers provided in the transistor 100 and the transistor102 and the insulating layer of the capacitor 104, and the like.

The FG portion of the memory element illustrated in FIG. 1A has afunction similar to that of a floating gate of a flash memory. However,a feature of the FG portion is essentially different from that of afloating gate of a flash memory. In the case of a flash memory, sincevoltage applied to a control gate is high, it is necessary to keep aproper distance between memory elements in order to prevent thepotential from affecting a floating gate of a memory element of theadjacent cell. Providing a proper distance between the memory elementsas described prevents high integration of a memory device.

Furthermore, in the flash memory, an insulating layer deteriorates bytunneling current, and the number of times of rewriting operations isrestricted.

The memory element illustrated in FIG. 1A operates with switching of thetransistors, and injection of electric charges by tunneling current isnot performed, which is different from the flash memory. That is, a highelectrical field for charge injection is not necessary unlike a flashmemory. Thus, there is no concern about effect of high electrical fieldfrom the control gate on the memory element of the adjacent cell, andhigher integration can be achieved as compared to the conventional one.Moreover, since a high electric field is unnecessary, a booster circuitis unnecessary at least for the memory element. Therefore, a large-sizedperipheral circuit is not necessary, and the frame of a memory devicecan be narrowed.

In the flash memory, electric charges travel in a gate insulating layer(a tunnel insulating film) during writing operation, so thatdeterioration of the gate insulating layer cannot be avoided. Incontrast, the memory element illustrated in FIG. 1A, data is written byswitching operation of a writing transistor; there is no cause ofdeterioration of the gate insulating layer. This means that there is nolimit on the number of times of writing in principle and writingdurability is very high. That is, the memory element illustrated in FIG.1A has higher durability and reliability than the flash memory. Forexample, in the memory element illustrated in FIG. 1A, writing operationcan be performed 1×10⁹ times (one billion times) or more, furtherpreferably, 1×10¹¹ (one hundred billion times).

In the case where the relative permittivity ε_(r1) of the insulatinglayer in the capacitor 104 is larger than or equal to the relativepermittivity ε_(r2) of the insulating layer in the transistor 100, it ispreferable that the following conditions be satisfied; S₁ is smallerthan or equal to twice S₂ (2S₂≧S₁, further preferably, S₁ is smallerthan or equal to S₂) where S₁ indicates an area of the capacitor 104 andS₂ indicates an area of a capacitor in the transistor 100; and thecapacitance value C₂ is lower than the capacitance value C₁. This isbecause higher integration can be realized. For example, a stack of afilm formed of a high-k material such as hafnium oxide and a film formedof an oxide semiconductor is used for the insulating layer in thecapacitor 104 so that ε_(d) can be 10 or more, preferably 15 or more;silicon oxide is used for the insulating layer of a capacitor in thetransistor 100 so that ε_(r2) can be 3 to 4.

Note that, although description here is made on the case of using ann-channel transistor in which electrons are majority carriers, ap-channel transistor in which holes are majority carriers may be used.

FIG. 1B is a top view illustrating an example of a specific structure ofthe memory element of FIG. 1A. FIG. 1C is a cross-sectional view takenalong line X-Y of FIG. 1B.

In FIG. 1C, the transistor 100 and the capacitor 104 are provided over asubstrate 116. The transistor 100 and the capacitor 104 are covered withan insulating layer, and the insulating layer is planarized by chemicalmechanical polishing (CMP) treatment or the like, so that the gateelectrode of the transistor 100 and the one of the electrodes of thecapacitor 104 are exposed. The other of the source and drain electrodesof the transistor 102 is provided over the exposed gate electrode of thetransistor 100 and the one of the electrodes of the capacitor 104. Notethat the transistor 100 here is a p-channel transistor, but it is notlimited thereto.

As illustrated in FIG. 1C, part of the same layer as the gate electrodeof the transistor 100 (part functioning as a back gate of the transistor102) overlaps with at least a portion functioning as a channel formationregion in the semiconductor layer of the transistor 102. The partfunctioning as the back gate of the transistor 102 and the semiconductorlayer of the transistor 102 are provided so that an insulating layerprovided over the transistor 100 is sandwiched therebetween. Thisinsulating layer is a portion of the insulating layer which has beenprovided over the transistor 100 and left after the planarizationtreatment, due to a lack of the thickness of a semiconductor layer ofthe transistor 100. As described above, the transistor in an upperportion and the back gate are provided with the insulating layer whichis left after the planarization treatment and interposed therebetween,and the back gate is formed of part of the same layer as the gateelectrode of the transistor in a lower portion, which are one offeatures of the memory element that is one embodiment of the presentinvention. In such a manner, the back gate of the transistor in theupper portion is formed of the same layer as the gate electrode of thetransistor in a lower portion, whereby the back gate electrode of thetransistor in an upper portion can be provided without an increase inthe number of manufacturing steps. Note that in this specification andthe like, the term “the same layer as A” indicates a layer formed fromthe same material in the same step as those of A.

The off-state current of the transistor 102 per micrometer of channelwidth at temperature in use (e.g., 25° C.) is 100 zA or less, preferably10 zA or small, further preferably 1 zA or less, still furtherpreferably 100 yA or less. Such a low off-state current can be achievedwith use of an oxide semiconductor for the transistor 102. Note that theoff-state current may be lower than the measurement limit.

In addition, by using an oxide semiconductor in the channel formationregion of the transistor 102, the subthreshold swing (S value) isreduced, so that the switching rate can be sufficiently high. Thus, inthe transistor 102 whose channel formation region is formed using anoxide semiconductor, rising of a writing pulse given to the FG portioncan be very sharp.

As described above, since the off-state current of the transistor 102 isdecreased, the amount of electric charges stored in the FG portion canbe reduced. Furthermore, operation speed of writing data and erasingdata can be increased; thus, rewriting data can be performed at highspeed.

As for the transistor 100, it is preferable to use a transistor whichoperates at high speed in order to increase the reading rate. Forexample, it is preferable to use a transistor with a switching rate of 1nanosecond or faster as the transistor 100.

Writing data is performed as follows: the transistor 102 is turned on;potential is supplied to the FG portion where the other of the sourceand drain electrodes of the transistor 102, the one of the electrodes ofthe capacitor 104, and the gate electrode of the transistor 100 areelectrically connected; and then the transistor 102 is turned off, sothat the predetermined amount of electric charges are held in the FGportion. Here, the off-state current of the transistor 102 is muchdecreased; thus, the electric charges supplied to the FG portion areheld for a long time. For example, when the off-state current is lowenough to be regarded as substantially zero, refresh operation is notneeded, or even when the refresh operation is performed, the frequencyof refresh operation can be drastically low (e.g., about once a month ora year), so that power consumed by the memory element can besignificantly reduced.

Note that in the memory element of FIGS. 1A to 1C, by overwriting data,data can be directly rewritten. Therefore, the memory element does notneed erasing operation which is necessary in a flash memory and thelike, so that a decrease in operation speed due to erasing operation canbe prevented.

The maximum value of the voltage applied to the memory element of FIGS.1A to 1C (the difference between the highest potential and the lowestpotential applied to respective terminals of the memory element at thesame time) is 5 V or lower, preferably 3 V or lower in one memoryelement, in the case where tow-stage (one bit) data is written.

Further, the oxide semiconductor used for the transistor 102 has anenergy gap as large as 3.0 eV to 3.5 eV, which is considered to be oneof main factors of low off-state current of the transistor 102.

The oxide semiconductor used in the transistor 102 has very fewthermally excited carriers; thus, even under a high-temperatureenvironment at 150° C., current-voltage characteristics of the memoryelement are not degraded.

For the transistor 102, it is preferable to use an intrinsic (i-type) orsubstantially intrinsic oxide semiconductor which is highly purified byremoval of an impurity so that an impurity serving as a carrier donorother than a main component of the oxide semiconductor is contained aslittle as possible.

As described, a highly purified oxide semiconductor layer includesextremely few carriers (close to zero), and the carrier concentrationthereof is lower than 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³,further preferably lower than 1×10¹¹/cm³. This is considered to be oneof factors of low off-state current of the transistor 102.

Such a highly-purified oxide semiconductor is extremely sensitive to aninterface level and interface charge; therefore, an interface betweenthe oxide semiconductor layer and the gate insulating layer isimportant. Thus, the gate insulating layer which is in contact with thehighly purified oxide semiconductor needs high quality.

The gate insulating layer formed by, for example, high-density plasmaCVD using microwave (for example, a frequency of 2.45 GHz) can be adense layer with high withstand voltage, which is preferable. The highlypurified oxide semiconductor and the high-quality gate insulating layerare provided to be in close contact with each other, so that theinterface state density can be reduced and favorable interfacecharacteristics can be obtained.

It is needless to say that another film formation method such as asputtering method or a plasma CVD method can be employed as long as ahigh-quality insulating layer can be formed as a gate insulating layer.

As an oxide semiconductor used in the transistor 102, the followingmetal oxide can be used: four-component metal oxide such as anIn—Sn—Ga—Zn—O-based oxide semiconductor; three-component metal oxidesuch as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-basedoxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, or a Sn—Al—Zn—O-based oxide semiconductor; two-componentmetal oxide such as an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, or an In—Ga—O-basedoxide semiconductor; a single component metal oxide such as anIn—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, or aZn—O-based oxide semiconductor; or the like. Further, silicon oxide maybe contained in the above oxide semiconductor. Here, for example, anIn—Ga—Zn—O-based oxide semiconductor means an oxide film containingindium (In), gallium (Ga), and zinc (Zn), and there is no particularlimitation on the composition ratio thereof. Further, theIn—Ga—Zn—O-based oxide semiconductor may contain an element other thanIn, Ga, and Zn.

For the oxide semiconductor film in the transistor 102, a thin film ofan oxide semiconductor represented by the chemical formula,InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, Ga and Co, or the like. In addition, the above oxidesemiconductor thin film may contain silicon oxide.

The oxide thin film can be formed by a sputtering method. Here, with useof an oxide target whose composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1[molar ratio], an In—Ga—Zn—O film can be formed, for example.Alternatively, an oxide target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] may be used.

Note that here, for example, an In—Ga—Zn—O film means an oxide filmcontaining In, Ga, and Zn, and there is no particular limitation on thecomposition ratio thereof.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target therefore has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

The filling factor of the oxide target is greater than or equal to 90%and less than or equal to 100%, preferably greater than or equal to 95%and less than or equal to 99.9%. With use of an oxide target with highfilling factor, an oxide semiconductor film which is a dense film can beformed.

Moreover, the oxide semiconductor film is preferably formed by asputtering method in a rare gas atmosphere, an oxygen atmosphere, or amixed atmosphere of a rare gas and oxygen. Further, a high-purity gasfrom which an impurity such as hydrogen, water, hydroxyl, or hydride isremoved is preferably used as a sputtering gas used in formation of theoxide semiconductor film.

FIG. 2 illustrates a structural example of a memory device in which thememory elements described with FIGS. 1A to 1C are provided in matrix, asa memory device which is one embodiment of the present invention.Although FIG. 2, for simplicity, illustrates a structure where thememory elements are arranged in matrix of 2 (rows) (in a horizontaldirection)×2 (columns) (in a vertical direction), a memory device inwhich memory elements are arranged in matrix of m (rows) (in ahorizontal direction)×n (columns) (in a vertical direction) (m and n arenatural numbers) is described below.

In the memory device illustrated in FIG. 2, a plurality of memoryelements 120 are arranged in matrix of m (rows) (in a horizontaldirection)×n (columns) (in a vertical direction) (m and n are naturalnumbers), and on a periphery thereof, a first driver circuit 121, asecond driver circuit 122, a third driver circuit 123, and a fourthdriver circuit 124 are provided. These driver circuits and the memoryelements 120 are connected with m word lines WL, m second signal linesS2, m back gate lines BW, n bit lines BL, n source lines SL, and n firstsignal lines S1. Here, the memory element 120 is the memory elementillustrated in FIG. 1A, which includes the transistor 100, thetransistor 102, and the capacitor 104.

The bit line BL corresponds to the second wiring 112 of the memoryelement illustrated in FIG. 1A, the source line SL corresponds to thefirst wiring 111 of the memory element illustrated in FIG. 1A, the firstsignal line S1 corresponds to the third wiring 113 of the memory elementillustrated in FIG. 1A, the second signal line S2 corresponds to thefourth wiring 114 of the memory element illustrated in FIG. 1A, and theword line WL corresponds to the fifth wiring 115 of the memory elementillustrated in FIG. 1A.

In other words, in the memory element 120, the one of the source anddrain electrodes of the transistor 100 is electrically connected to thesource line SL, the other of the source and drain electrodes of thetransistor 100 is electrically connected to the bit line BL. The one ofthe source and drain electrodes of the transistor 102 is electricallyconnected to the first signal line S1, and a gate electrode of thetransistor 102 is electrically connected to the second signal line S2. Agate electrode of the transistor 100 and the other of the source anddrain electrodes of the transistor 102 are electrically connected to theone of the electrodes of the capacitor 104. The other electrode of thecapacitor 104 is electrically connected to the word line WL. A back gateBG provided in the transistor 102 is electrically connected to the backgate line BW.

The memory elements 120 are connected in parallel between the sourcelines SL and the bit lines BL. For example, the memory element 120 of ani-th row and a j-column (i,j) (i is an integer which is larger than orequal to 1 and smaller than or equal to m, and j is an integer which islarger than or equal to 1 and smaller than or equal to n) is connectedto the source line SL(j), the bit line BL(j), the first signal lineS1(j), the word line WL(i), the second signal line S2(i), and the backgate line BW (i).

The source lines SL and the bit lines BL are connected to the firstdriver circuit 121, the second signal lines S2 and the back gate linesBW are connected to the second driver circuit 122, the first signallines S1 are connected to the third driver circuit 123, and the wordlines WL are connected to the fourth driver circuit 124.

Note that the first driver circuit 121, the second driver circuit 122,the third driver circuit 123 and the fourth driver circuit 124 areindependently provided here; however, the periphery circuit structure isnot limited to this, a decoder having one or more functions may also beused.

Next, the writing operation and the reading operation of the memorydevice illustrated in FIG. 2 are described with reference to a timingchart of FIG. 3.

Although operation of the memory device of two rows by two columns willbe described for simplification, the present invention is not limited tothis.

In FIG. 3, S1(1) and S1(2) are potentials of the first signal lines S1;S2(1) and S2(2) are potentials of the second signal lines S2; BL(1) andBL(2) are potentials of the bit lines BL; WL(1) and WL(2) are potentialsof the word lines WL; and SL(1) and SL(2) are potentials of the sourcelines SL.

Is described the case where data is written to the memory element 120(1,1) and the memory element 120 (1,2) of the first row and data is readfrom the memory element 120 (1,1) and the memory element 120 (1,2) ofthe first row. Note that the description below is about the case wheredata written to the memory element 120 (1,1) is “1” (which can supply ahigh level electric charge to the FG portion) and data written to thememory element (1,2) is “0” (which can supply a low level electriccharge to the FG portion).

First, the writing will be described. In a writing period of the firstrow, a potential V_(H) is supplied to the second signal line S2(1) ofthe first row so that the second transistors 102 of the first row areon. Further, a potential of 0 V is supplied to the second signal lineS2(2) of the second row so that the second transistors 102 of the rowother than the first row are turned off.

Next, the potential V₂ and the potential 0 V are applied to the firstsignal line S1(1) of the first column and the first signal line S1(2) ofthe second column, respectively.

As a result, the FG portion of the memory element (1,1) is supplied withthe potential V₂, and the FG portion of the memory element (1,2) issupplied with 0V. Here, the potential V₂ is higher than the thresholdvoltage of the transistor. Then, the potential of the second signal lineS2(1) of the first row is set to the potential 0 V, so that thetransistors 102 of the first row are turned off. Thus, the writing iscompleted.

Note that the word lines WL(1) and WL(2) are at a potential of 0 V.Further, before the potential of the first signal line S1(1) of thefirst column is changed, the potential of the second signal line S2(1)of the first row is set to 0 V. After the writing, the threshold voltageof a memory element is V_(w0) in the case where data “0” has beenwritten and V_(w1) in the case where data “1” has been written, assumingthat a terminal electrically connected to the word line WL is a controlgate electrode, the source electrode of the transistor 100 is a sourceelectrode, and the drain electrode of the transistor 102 is a drainelectrode, in the memory element. Here, the threshold voltage of thememory element means a voltage of a terminal connected to the word lineWL, which changes resistance between the source electrode and the drainelectrode of the transistor 100. Note that V_(w0)>0>V_(w1) is satisfied.

Then, the reading will be described. In a reading period of the firstrow, a potential 0 V and the potential V_(L) are supplied to the wordline WL(1) of the first row and the word line WL(2) of the second row,respectively. The potential V_(L) is lower than the threshold voltageV_(w1). When the word line WL(1) is set to 0 V, in the first row, thetransistor 100 of the memory element 120 in which data “0” is held isturned off, and the transistor 100 of the memory element 120 in whichdata “1” is held is turned on. When the word line WL(2) is at thepotential V_(L), in the second row, the transistor 100 of the memoryelement 120 in which either data “0” or data “1” is held is off.

Next, a potential of 0 V is supplied to the source line SL(1) of thefirst column and the source line SL(2) of the second column.

As a result, the resistance between the bit line BL(1) and the sourceline SL(1) is low because the first transistor 100 in the memory element120 (1,1) is on, and the resistance between the bit line BL(2) and thesource line SL(2) is high because the transistor 100 in the memoryelement 120 (1,2) is off. A read-out circuit connected to the bit lineBL(1) and the bit line BL(2) can read data on the basis of a differencein resistance between the bit lines BL.

Further, a potential of 0 V and the potential V_(L) are supplied to thesecond signal line S2(1) and the second signal line S2(2), respectively,so that all the transistors 102 are off. The potentials of the FGportions of the first row are 0 V or V₂; thus, all the transistors 102can be turned off by setting the potential of the second signal lineS2(1) to 0 V. On the other hand, the potentials of the FG portions ofthe second row are lower than the potential at the time directly afterdata writing if the potential V_(L) is supplied to the word line WL(2).Therefore, in order to prevent the transistor 102 from being turned on,the potential of the second signal line S2(2) is set to low similarly tothe potential of the word line WL(2). Thus, all the transistors 102 canbe turned off.

During the above operation, the back gate line BW(1) and the back gateline BW(2) may have high potential.

A read-out circuit is used for reading data. FIG. 4A illustrates anexample of a read-out circuit. The read-out circuit illustrated in FIG.4A includes a transistor and a sense amplifier. The potential V_(dd) isapplied to one of source and drain of a transistor, and the other of thesource and drain of the transistor is connected to a plus terminal ofthe sense amplifier and a bit line. The bias potential V_(bias) isapplied to a gate of the transistor. The bias potential V_(bias) ishigher than 0 and lower than V_(dd). Further, the reference potentialV_(ref) is input to a minus terminal of the sense amplifier.

In the case where the memory element has low resistance, the potentialinput to the plus terminal of the sense amplifier is lower than thereference potential V_(ref) and the sense amplifier outputs data “1”. Onthe other hand, in the case where the memory element has highresistance, the potential input to the plus terminal of the senseamplifier is higher than the reference potential V_(ref) and the senseamplifier outputs data “0”. When the transistor 100 of the memoryelement (1,1) is on, resistance between the bit line BL(1) and thesource line SL(1) is low. Thus, an input of the sense amplifier is lowpotential and an output D(1) becomes High. Meanwhile, when thetransistor 100 of the memory element (1,2) is off, resistance betweenthe bit line BL(2) and the source line SL(2) is high; thus, an input ofthe sense amplifier is high potential and an output D(2) becomes Low.

FIG. 4B illustrates another example of the read-out circuit. Theread-out circuit illustrated in FIG. 4B includes a transistor and aclocked inverter. The potential V_(dd) is applied to one of source anddrain of the transistor, and the other of the source and drain of thetransistor is electrically connected to an input of the clocked inverterand a bit line. The potential V_(dd) is also applied to a gate of thetransistor.

An output potential in the case of using the read-out circuitillustrated in FIG. 4B is described. When the transistor 100 of thememory element (1,1) is on, resistance between the bit line BL(1) andthe source line SL(1) is low. Thus, the input of the clocked inverterhas low potential and an output D(1) becomes High. Meanwhile, when thetransistor 100 of the memory element (1,2) is off, resistance betweenthe bit line BL(2) and the source line SL(2) is high, and thus, theinput of the clocked inverter has high potential and an output D(2)becomes Low.

The structure of the read-out circuit is not limited to those in FIGS.4A and 4B. For example, the read-out circuit may include a prechargecircuit or a bit line for reference may be connected instead of applyingthe reference potential V_(ref).

The memory device is not limited to that illustrated in FIG. 2 but mayhave a different structure including the memory element illustrated inFIGS. 1A to 1C, from that in FIG. 2.

Hereinafter, a method for manufacturing the memory element 120 isdescribed with reference to FIGS. 5A to 5H, FIGS. 6A to 6G, and FIGS. 7Ato 7D. First, an example of a method for manufacturing an SOI substrateprovided with the transistor 100 is described with reference to FIGS. 5Ato 5H.

First, a base substrate 150 is prepared (see FIG. 5A). As the basesubstrate 150, a substrate made of an insulator can be used.Specifically, as examples thereof, a glass substrate, a quartzsubstrate, a ceramic substrate and a sapphire substrate can be given.

Alternatively, a semiconductor substrate such as a single crystalsilicon substrate or a single crystal germanium substrate may be used asthe base substrate 150. In the case of using a semiconductor substrateas the base substrate 150, the temperature limitation for heat treatmentis eased compared with the case of using a glass substrate or the like;thus, a high-quality SOI substrate is easily obtained. Here, as asemiconductor substrate, a solar grade silicon (SOG-Si) substrate or thelike may be used. Alternatively, a polycrystalline semiconductorsubstrate may be used. In the case of using a SOG-Si substrate, apolycrystalline semiconductor substrate, or the like, manufacturing costcan be reduced as compared to the case of using a single crystal siliconsubstrate or the like.

In this embodiment, a glass substrate is used for the base substrate150. Using a glass substrate as the base substrate 150 enables costreduction.

Next, a nitrogen-containing layer 152 (e.g., a layer including aninsulating film containing nitrogen, such as a silicon nitride film) isformed on a surface of the base substrate 150 (FIG. 5B). Thenitrogen-containing layer 152 functions as a layer for bonding a singlecrystal semiconductor layer (a bonding layer). The nitrogen-containinglayer 152 also functions as a barrier layer for preventing an impuritycontained in the base substrate, such as sodium (Na), from diffusinginto the single crystal semiconductor layer.

Here, it is preferable that the nitrogen-containing layer 152 havesurface planarity at a certain level because the nitrogen-containinglayer 152 is used to function as a bonding layer. Specifically, thenitrogen-containing layer 152 is formed such that it has an averagesurface roughness (arithmetic mean deviation) of 0.5 nm or less and aroot-mean-square surface roughness of 0.60 nm or less, preferably anaverage surface roughness of 0.35 nm or less and a root-mean-squaresurface roughness of 0.45 nm or less. Note that the average surfaceroughness and root-mean-square surface roughness can be measured, forexample, in a region of 10 square micrometers.

Next, a bond substrate 160 is prepared. A single crystal semiconductorsubstrate (e.g., a single crystal silicon substrate) is used as the bondsubstrate 160 (FIG. 5C). However, the bond substrate 160 is not limitedthereto.

An oxide film 162 is formed on a surface of the bound substrate 160(FIG. 5D). In view of removal of contamination, it is preferable thatthe surface of the bond substrate 160 be cleaned with a hydrochloricacid/hydrogen peroxide mixture (HPM) or the like before formation of theoxide film 162. The oxide film 162 can be formed with, for example, asingle layer of a silicon oxide film, a silicon oxynitride film, or thelike or a stack of any of the above films. The oxide film 162 ispreferably formed using organosilane such as tetraethoxysilane(abbreviation: TEOS, chemical formula: Si(OC₂H₅)₄).

Next, the bond substrate 160 that is a single crystal semiconductorsubstrate is irradiated with ions accelerated by an electrical field sothat the ions are added to the bond substrate 160, whereby an embrittledregion 164 is formed at a predetermined depth in the bond substrate 160that is a single crystal semiconductor substrate (FIG. 5E). The ionirradiation treatment is performed with an ion-doping apparatus or anion-implantation apparatus. In the treatment, a gas containing hydrogenis used as a source gas. As for ions used for the irradiation, theproportion of H₃ ⁺ is preferably set high. This is because efficiency ofion irradiation can be improved.

Note that the added ion is not limited to a hydrogen ion, and an ion ofhelium or the like may be added. Further, the added ion is not limitedto one kind, and plural kinds of ions may be added. For example, in thecase of performing irradiation with hydrogen and helium concurrentlyusing an ion doping apparatus, the number of steps can be reduced ascompared to the case of performing irradiation of hydrogen and helium inseparate steps, and increase in surface roughness of a single crystalsemiconductor layer to be formed later can be further suppressed.

The depth at which the embrittled region 164 is formed is determined bythe kinetic energy, mass, charge amount, or incidence angle of the ions,or the like, which is almost the same as the average penetration depthof the ions. Therefore, the thickness of a single crystal semiconductorlayer to be separated from the bound substrate 160 that is the singlecrystal semiconductor substrate can be controlled by the depth at whichthe ions are added.

Next, the surface of the base substrate 150 and the bond substrate 160are disposed to face each other, and the surface of thenitrogen-containing layer 152 and the surface of the oxide film 162 aredisposed in close contact with each other. In such a manner, the basesubstrate 150 and the bond substrate 160 are bonded to each other (FIG.5F).

When the base substrate 150 and the bond substrate 160 are bonded, it ispreferable that a pressure greater than or equal to 0.001 N/cm² and lessthan or equal to 100 N/cm² be applied to one part of the base substrate150 or the bond substrate 160. By applying a pressure in such a manner,the nitrogen-containing layer 152 and the oxide film 162 are bonded atthe portion where they are in contact with each other, and the bondingspontaneously spreads to the entire area. This bonding is performedunder the action of the Van der Waals force or hydrogen bonding and canbe performed at room temperature.

After the base substrate 150 and the bond substrate 160 are bonded, heattreatment may be performed in order to further strengthen the bond. Thisheat treatment is performed at a temperature at which separation at theembrittled region 164 does not occur (for example, higher than or equalto room temperature and lower than 400° C.). Alternatively, thenitrogen-containing layer 152 and the oxide film 162 may be bonded toeach other while being heated at a temperature within this range.

Next, the bond substrate 160 is divided along the embrittled region 164by heat treatment, so that a single crystal semiconductor layer 166 isformed over the base substrate 150 with the nitrogen-containing layer152 and the oxide film 162 interposed therebetween (FIG. 5G).

The temperature of the heat treatment for separation is preferably lowso as to suppress generation of roughness on the surface of the singlecrystal semiconductor layer 166. The temperature of the heat treatmentfor separation may be, for example, higher than or equal to 300° C. andlower than or equal to 600° C., and the temperature lower than or equalto 500° C. (higher than or equal to 400° C.) is more effective.

Note that after the bond substrate 160 is separated, the single crystalsemiconductor layer 166 may be subjected to heat treatment at 500° C. orhigher so that the concentration of hydrogen remaining in the singlecrystal semiconductor layer 166 is reduced.

Next, a surface of the single crystal semiconductor layer 166 isirradiated with laser light, whereby a semiconductor layer 168 where theflatness of the surface planarity is improved and the number of defectsis reduced is formed. Note that instead of the laser light irradiationtreatment, heat treatment may be performed.

Although the irradiation treatment with the laser light described ishere performed just after the heat treatment for separation, theirradiation treatment with the laser light may be performed after aregion having many defects in the surface of the single crystalsemiconductor layer 166 is removed by etching or the like.Alternatively, the irradiation treatment with the laser light may beperformed after a level of planarity of the surface of the crystalsemiconductor layer 166 is improved.

Through the above steps, the SOI substrate including the semiconductorlayer 168 can be obtained (FIG. 5H).

Next, a method for manufacturing a transistor with the above SOIsubstrate is described with reference to FIGS. 6A to 6G.

First, the semiconductor layer 168 illustrated in FIG. 6A is processedto have an island shape, so that a semiconductor layer 170 is formed(FIG. 6B).

Note that before or after processing the semiconductor layer 168 intothe island-shaped layer, an impurity element imparting n-typeconductivity or an impurity element imparting p-type conductivity may beadded to the semiconductor layer 168 or the semiconductor layer 170 inorder to control the threshold voltage of the transistor. In the casewhere a material of the semiconductor layer 168 is silicon, P, As, orthe like can be used as an impurity element imparting n-typeconductivity, and alternatively, B, Al, Ga, or the like can be used asan impurity element imparting p-type conductivity, for example.

Next, an insulating layer 172 is formed to cover the semiconductor layer170 (FIG. 6C). The insulating layer 172 functions as a gate insulatinglayer later.

Next, a conductive layer is formed over the insulating layer 172; then,the conductive layer is selectively etched so that a gate electrode 174is formed to overlap with the semiconductor layer 170 (FIG. 6D). In thisstep, the one of the electrodes of the capacitor 104 and the back gateBG of the transistor 102 as well as the gate electrode 174 can be alsoformed.

Next, with use of the gate electrode 174 as a mask, an impurity elementimparting one conductivity type is added to the semiconductor layer 170,so that an impurity region 176 and a channel formation region 178 areformed (FIG. 6E). Note that in order to form a p-channel transistor inthis embodiment, an impurity element such as B or Al is added; however,in the case of forming an n-channel transistor, P or As may be added.The impurity region 176 functions as a source region or a drain region.

Although not illustrated here, a sidewall insulating layer may be formedon the side surfaces of the gate electrode 174.

Then, an interlayer insulating layer 180 is formed so as to cover thecomponents formed through the above steps (FIG. 6F). The interlayerinsulating layer 180 may be formed using a material including aninorganic insulating material such as silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, hafnium oxide, aluminum oxide,or tantalum oxide; or an organic insulating material such as polyimideor acrylic. The interlayer insulating layer 180 may have a stackedstructure.

Next, a surface of the interlayer insulating layer 180 is planarized byCMP treatment, etching treatment, or the like (FIG. 6G). By the CMP oretching treatment, the gate electrode 174 is exposed.

Through the above-described steps, the transistor 100 with use of theSOI substrate can be formed. Since such a transistor 100 can operate athigh speed, with such a transistor 100, a logic circuit (also referredto as an arithmetic circuit) or the like can be constituted. In otherwords, the transistor 100 can be used for a driver circuit of a memorydevice or the like.

Note that the structure of the transistor 100 is not limited to thatillustrated in FIG. 6G, and an electrode, a wiring, an insulating layer,and the like are additionally formed in the transistor.

Next, a method for forming the transistor 102 over the transistor 100 isdescribed with reference to FIGS. 7A to 7D.

First, a conductive layer is formed over the interlayer insulating layer180 which has been subjected to planarization treatment as illustratedin FIG. 6G, and the conductive layer is processed into a conductivelayer 182 (FIG. 7A). There is no particular limitation on a material anda formation method of the conductive layer 182. The conductive layer 182is provided at least in a needed region so as to be in contact with theexposed portion of the gate electrode 174.

Next, a semiconductor film is formed over the conductive layer 182, andthe semiconductor film is processed into a semiconductor layer 184 (FIG.7B). Here, the semiconductor layer 184 is formed using an oxidesemiconductor.

Dehydration or dehydrogenation may be performed by performing preheatingbefore the semiconductor film is formed.

It is preferable that remaining moisture and hydrogen in a depositionchamber be sufficiently removed before the semiconductor film is formed.That is, before formation of the semiconductor film, evacuation ispreferably performed with an entrapment vacuum pump such as a cryopump,an ion pump, or a titanium sublimation pump.

Next, first heat treatment may be performed on the oxide semiconductorlayer. Here, the first heat treatment is performed in order to dehydrateor dehydrogenate the oxide semiconductor layer. The temperature of thefirst heat treatment is higher than or equal to 400° C. and lower thanor equal to 750° C., preferably higher than or equal to 400° C. andlower than the strain point of the substrate. For example, the oxidesemiconductor layer is subjected to heat treatment in a nitrogenatmosphere at 450° C. for one hour, and then water or hydrogen isprevented from entering the oxide semiconductor layer, so that adehydrated or dehydrogenated oxide semiconductor layer can be formed.Note that timing of the first heat treatment is not limited to this, andthe first heat treatment may be performed in a later step.

Then, an insulating layer 186 is formed to cover the semiconductor layer184 (FIG. 7C). The insulating layer 186 functions as a gate insulatinglayer.

Next, second heat treatment is performed in an inert gas (includingnitrogen) atmosphere or oxygen gas atmosphere (preferably at 200° C. to400° C. inclusive, e.g. 250° C. to 350° C. inclusive). In thisembodiment, the second heat treatment is performed in a nitrogenatmosphere at 300° C. for one hour. In the second heat treatment, partof the oxide semiconductor layer (a channel formation region) is heatedin a state of being in contact with the insulating layer 186. In thecase where oxygen is supplied to the oxide semiconductor layer, theinsulating layer 186 is preferably formed using a material containingoxygen.

Note that the oxide semiconductor layer may have either an amorphousstructure or a structure with crystallinity. In the case where the oxidesemiconductor layer has crystallinity, the oxide semiconductor layer maybe formed by two deposition steps and heat treatment may be performedtwice with the two deposition.

Then, a conductive layer 188 is formed over the insulating layer 186 soas to overlap with at least a portion functioning as the channelformation region of the semiconductor layer 184.

Through the above steps, the transistor 102 can be formed.

Note that the structure of the transistor 102 is not limited to thatillustrated in FIG. 7D, and an electrode, a wiring, an insulating layer,and the like may be additionally formed in the transistor.

Embodiment 2

In this embodiment, a memory element which is an embodiment of thepresent invention and is different from that of Embodiment 1 will bedescribed. Specifically, an embodiment in which a transistor in a lowerportion has a structure similar to the transistor in an upper portion,which is a different point from Embodiment 1, will be described withreference to FIGS. 8A to 8C.

A memory element illustrated in FIG. 8A includes a transistor 200, atransistor 202, and a capacitor 204. In FIG. 8A, one of source and drainelectrodes of the transistor 200 is electrically connected to a firstwiring 211, and the other of the source and drain electrodes of thetransistor 200 is electrically connected to the second wiring 212. Oneof source and drain electrodes of the transistor 202 is electricallyconnected to a third wiring 213, and a gate electrode of the transistor202 is electrically connected to a fourth wiring 214. A gate electrodeof the transistor 200 and the other of the source and drain electrodesof the transistor 202 are electrically connected to one of electrodes ofthe capacitor 204. The other electrode of the capacitor 204 iselectrically connected to a fifth wiring 215. The transistor 200 isprovided with a back gate BG1 functioning as another gate electrode. Thetransistor 202 is provided with a back gate BG2 functioning as anothergate electrode.

FIG. 8B is a top view illustrating an example of a specific structure ofthe memory element of FIG. 8A. FIG. 8C is a cross-sectional view takenalong line X-Y of FIG. 8B.

As illustrated in FIG. 8B, the transistor 202 can be the same transistoras the transistor 102 of FIGS. 1A to 1C.

However, the transistor 200 is different from the transistor 100 and isa transistor which is formed similarly to the transistor 202. In otherwords, it is preferable for the transistor 200 to include an oxidesemiconductor layer which is used for a channel formation region.

The capacitor 204 includes part of the same layer as the source anddrain electrode layers of the transistor 200 and part of the same layeras the gate electrode of the transistor 200.

Further, the capacitor 204 may be constituted by including part of thesame layer as the gate electrode provided on the substrate side (a layerto be the back gate of the transistor 200).

In FIG. 8C, the transistor 200 and the capacitor 204 are provided over asubstrate 216. The transistor 200 and the capacitor 204 are covered withan insulating layer, and the insulating layer is subjected toplanarization treatment using CMP or the like, so that the gateelectrode of the transistor 200 and the one of the electrodes of thecapacitor 204 are exposed. The other of the source and drain electrodesof the transistor 202 is provided on the exposed gate electrode of thetransistor 200 and the one of the electrodes of the capacitor 204.

As illustrated in FIG. 8C, part of the same layer as the gate electrodeof the transistor 200 (part functioning as the back gate of thetransistor 202) overlaps with at least a region functioning as a channelformation region in a semiconductor layer of the transistor 202. Thepart functioning as the back gate of the transistor 202 and thesemiconductor layer of the transistor 202 are provided so that aninsulating layer over the transistor 200 is sandwiched therebetween.This insulating layer is a portion of the insulation layer which hasbeen provided over the transistor 200 and left after the planarizationtreatment, due to a lack of the thickness of the semiconductor layer ofthe transistor 200. As described above, the transistor in an upperportion and the back gate are provided with the insulating layer whichis left after the planarization treatment and interposed therebetween,and the back gate is formed of part of the same layer as the gateelectrode of the transistor in a lower portion, which are one offeatures of the memory element that is one embodiment of the presentinvention. In such a manner, the back gate of the transistor in an upperportion is formed of part of the same layer as the gate electrode of thetransistor in a lower portion, whereby the back gate of the transistorin an upper portion can be provided without an increase in the number ofmanufacturing steps.

Although FIG. 8C illustrates a structure in which both the transistor200 and the transistor 202 are provided with the back gates, thestructure of the memory element is not limited thereto. A structurewhere the transistor 200 is not provided with a back gate may beemployed.

Embodiment 3

In this embodiment, an element which is an embodiment of the presentinvention and different from those of Embodiment 1 and Embodiment 2 willbe described. Specifically, an inversion element which can bemanufactured in a manner similar to that of Embodiment 1 will bedescribed with reference to FIGS. 9A to 9C.

An inversion element illustrated in FIG. 9A included a transistor 300and a transistor 302. In FIG. 9A, one of source and drain electrodes ofthe transistor 302 is electrically connected to a fourth wiring 314 atground potential V_(ss), and the other of the source and drainelectrodes of the transistor 302 is electrically connected to one ofsource and drain electrodes of the transistor 300 and a second wiring312. The other of the source and drain electrodes of the transistor 300is electrically connected to a third wiring 313 at power supplypotential V_(dd). A gate electrode of the transistor 302 is connected toa gate electrode of the transistor 300 and a first wiring 311. Thetransistor 302 is provided with a back gate BG functioning as anothergate electrode.

FIG. 9B is a top view illustrating an example of a specific structure ofthe inversion element of FIG. 9A. FIG. 9C is a cross-sectional viewtaken along X-Y of FIG. 9B.

As illustrated in FIG. 9B, the transistor 300 can be the same transistoras the transistor 100 of FIGS. 1A to 1C. The transistor 302 can be thesame transistor as the transistor 102 of FIGS. 1A to 1C.

In FIG. 9C, the transistor 300 is provided over a substrate 316. Thetransistor 300 is covered with an insulating layer, and the insulatinglayer is subjected to planarization treatment using CMP or the like, sothat the gate electrode of the transistor 300 is exposed. Over theexposed gate electrode of the transistor 300, part of the same layer asthe source and drain electrode layers of the transistor 302 is providedand is electrically connected to the gate electrode of the transistor302 through the wiring 311 (not illustrated in FIG. 9C). The transistor300 is a p-channel transistor here but is not limited thereto.

As illustrated in FIG. 9C, part of the same layer as the gate electrodeof the transistor 300 (part functioning as the back gate of thetransistor 302) overlaps with at least a portion functioning as achannel formation region in a semiconductor layer of the transistor 302.The part functioning as the back gate of the transistor 302 and thesemiconductor layer of the transistor 302 are provided so that aninsulating layer provided over the transistor 300 is sandwichedtherebetween. This insulating layer is a portion of the insulating layerwhich has been provided over the transistor 300 and left after theplanarization treatment, due to a lack of the thickness of thesemiconductor layer of the transistor 300. As described above, thetransistor in an upper portion and the back gate are provided with theinsulating layer which is left after the planarization treatment andinterposed therebetween, and the back gate is formed of part of the samelayer as the gate electrode of the transistor in a lower portion, whichare one of features of the inversion element that is one embodiment ofthe present invention. In such a manner, the back gate of the transistorin an upper portion is formed of part of the same layer as the gateelectrode of the transistor in a lower portion, whereby the back gate ofthe transistor in an upper portion can be provided without an increasein the number of manufacturing steps.

Embodiment 4

In this embodiment, an element which is an embodiment of the presentinvention and different those of from Embodiment 1 to Embodiment 3 willbe described. Specifically, an inversion element which can bemanufactured in a manner similar to that of Embodiment 2 will bedescribed with reference to FIGS. 10A to 10C.

An inversion element illustrated in FIG. 10A includes a transistor 400and a transistor 402. In FIG. 10A, one of source and drain electrodes ofthe transistor 402 is electrically connected to a fourth wiring 414 atground potential V_(ss), and the other of the source and drainelectrodes of the transistor 402 is electrically connected to one ofsource and drain electrodes of the transistor 400 and a second wiring412. The other of the source and drain electrodes of the transistor 400is electrically connected to a third wiring 413 at power supplypotential V_(dd). A gate electrode of the transistor 400 is connected tothe other of the source and drain electrodes of the transistor 400. Agate electrode of the transistor 402 is electrically connected to thefirst wiring 411. The transistor 400 is provided with a back gate BG1functioning as another gate electrode. The transistor 402 is providedwith a back gate BG2 functioning as another gate electrode.

FIG. 10B is a top view illustrating an example of a specific structureof the inversion element of FIG. 10A. FIG. 10C is a cross-sectional viewtaken along X-Y of FIG. 10B.

As illustrated in FIG. 10B, the transistor 402 can be the sametransistor as the transistor 302 of FIGS. 9A to 9C.

However, the transistor 400 is different from the transistor 300 and isa transistor which is formed similarly to the transistor 402. In otherwords, it is preferable for the transistor 402 to include an oxidesemiconductor layer which is used for a channel formation region.Further, the channel width of the transistor 402 is preferably muchlarger than that of the transistor 400, further preferably three timesor more that of the transistor 400, still further preferably five timesor more that of the transistor 400.

In FIG. 10C, the transistor 400 is provided over the substrate 416. Thetransistor 400 is covered with an insulating layer, and the insulatinglayer is subjected to planarization treatment using CMP or the like, sothat the gate electrode of the transistor 400 is exposed. Above theexposed gate electrode of the transistor 400, part of the same layer asthe source and drain electrode layers of the transistor 402 is providedand electrically connects the gate electrode of the transistor 400 andthe third wiring 413 (not illustrated in FIG. 10C).

As illustrated in FIG. 10C, part of the same layer as the gate electrodeof the transistor 400 (part functioning as the back gate of thetransistor 402) overlaps with at least a portion functioning as achannel formation region in a semiconductor layer of the transistor 402.The part functioning as the back gate of the transistor 402 and thesemiconductor layer of the transistor 402 are provided so that aninsulating layer provided over the transistor 400 is sandwichedtherebetween. This insulating layer is a portion of the insulating layerwhich has been provided over the transistor 400 and left after theplanarization treatment, due to a lack of thickness of the semiconductorlayer of the transistor 400. As described above, the transistor in anupper portion and the back gate are provided with the insulating layerwhich is left after the planarization treatment and interposedtherebetween, and the back gate is formed of part of the same layer asthe gate electrode of the transistor in a lower portion, which are oneof features of the inversion element that is one embodiment of thepresent invention. In such a manner, the back gate of the transistor inan upper portion is formed of part of the same layer as the gateelectrode of the transistor in a lower portion, whereby the back gate ofthe transistor in an upper portion can be provided without an increasein the number of manufacturing steps.

Embodiment 5

In this embodiment, an element which is an embodiment of the presentinvention and different from those of Embodiment 1 to Embodiment 4 willbe described. Specifically, a NAND gate which is one of logic gates andcan be manufactured in a manner similar to that of Embodiment 1 will bedescribed with reference to FIGS. 11A to 11C.

A memory element illustrated in FIG. 11A includes a transistor 500, atransistor 502, a transistor 504, and a transistor 506. In FIG. 11A, oneof source and drain electrodes of the transistor 500 is electricallyconnected to a fifth wiring 515 at power supply potential K_(dd) and oneof source and drain electrodes of the transistor 502. The other of thesource and drain electrodes of the transistor 500 is electricallyconnected to a third wiring 513, the other of the source and drainelectrodes of the transistor 502, and one of source and drain electrodesof the transistor 504. The other of the source and drain electrodes ofthe transistor 504 is electrically connected to one of source and drainelectrodes of the transistor 506. The other of the source and drainelectrodes of the transistor 506 is electrically connected to a fourthwiring 514 at ground potential V_(ss). A gate electrode of thetransistor 502 and a gate electrode of the transistor 504 are connectedto a first wiring 511. A gate electrode of the transistor 500 and a gateelectrode of the transistor 506 are connected to a second wiring 512.The transistor 504 is provided with a back gate BG1 functioning asanother gate electrode, and the transistor 506 is provided with a backgate BG2 functioning as another gate electrode.

FIG. 11B is a top view illustrating an example of a specific structureof the memory element of FIG. 11A. FIG. 11C is a cross-sectional viewtaken along X-Y of FIG. 11B.

As illustrated in FIG. 11B, each of the transistor 500 and thetransistor 502 can be the same transistor as the transistor 100illustrated in FIGS. 1A to 1C. Each of the transistor 504 and thetransistor 506 can be the same transistor as the transistor 102illustrated in FIGS. 1A to 1C.

In FIG. 11C, the transistor 502 is formed over a substrate 516. Thetransistor 502 is covered with an insulating layer, and the insulatinglayer is planarized by CMP treatment or the like, so that the gateelectrode of the transistor 502 is exposed. Over the exposed gateelectrode of the transistor 502, part of the same layer as the sourceand drain electrode layers of the transistor 504 and the transistor 506is provided, whereby the gate electrode of the transistor 502 and thefirst wiring 511 are electrically connected to each other with the samelayer (not illustrated in FIG. 11C). Although not illustrated, thetransistor 500 is electrically connected to the second wiring 512 in asimilar manner. Note that the transistor 500 and the transistor 502 arep-channel transistors here but not limited thereto.

Parts of the same layer as the gate electrodes of the transistor 500 andthe transistor 502 (parts functioning as the back gates of thetransistor 504 and the transistor 506) overlap with at least portionsfunctioning as channel formation regions in semiconductor layers of thetransistor 504 and the transistor 506. The parts functioning as the backgates of the transistor 504 and the transistor 506 and the semiconductorlayers of the transistor 504 and the transistor 506 are provided so thatan insulating layer provided over the transistor 500 and the transistor502 is sandwiched therebetween. This insulating layer is a portion ofthe insulating layer which has been provided over the transistor 500 andthe transistor 502 and left after the planarization treatment, due tothe thickness of the semiconductor layers of the transistor 500 and thetransistor 502. As described above, the transistors in an upper portionand the back gates are provided with the insulating layer which is leftafter the planarization treatment and interposed therebetween, and theback gates are formed of parts of the same layer as the gate electrodesof the transistors in a lower portion, which are one of features of thememory element that is one embodiment of the present invention. In sucha manner, the back gates of the transistors in an upper portion areformed of parts of the same layer as the gate electrodes of thetransistors in a lower portion, whereby the back gates of thetransistors in an upper portion can be provided without an increase inthe number of manufacturing steps.

Embodiment 6

In this embodiment, an element which is an embodiment of the presentinvention and different from those of Embodiment 1 to Embodiment 5 willbe described. Specifically, a NAND gate which is one of logic gates andcan be manufactured in a manner similar to that of Embodiment 2 will bedescribed with reference to FIGS. 12A to 12C.

A memory element illustrated in FIG. 12A includes a transistor 600, atransistor 602, and a transistor 604. In FIG. 12A, one of source anddrain electrodes of the transistor 600 is connected to a fourth wiring614 at power supply potential V_(dd), and the other of the source anddrain electrodes of the transistor 600 is connected to one of source anddrain electrodes of the transistor 602 and a third wiring 613. The otherof the source and drain electrodes of the transistor 602 is connected toone of source and drain electrodes of the transistor 604, and the otherof the source and drain electrodes of the transistor 604 is connected toa fifth wiring 615 at ground potential V_(ss). A gate electrode of thetransistor 600 is connected to the fourth wiring 614. A gate electrodeof the transistor 602 is connected to a first wiring 611. A gateelectrode of the transistor 604 is connected to a second wiring 612. Thetransistor 600 is provided with a back gate BG1 functioning as anothergate electrode. The transistor 602 is provided with a back gate BG2functioning as another gate electrode. The transistor 604 is providedwith a back gate BG3 functioning as another gate electrode.

FIG. 12B is a top view illustrating an example of a specific structureof the memory element of FIG. 12A. FIG. 12C is a cross-sectional viewtaken along X-Y of FIG. 12B.

As illustrated in FIG. 12B, the transistor 602 and the transistor 604can be the same transistors as the transistor 504 and the transistor 506of FIGS. 11A to 11C.

However, the transistor 600 is different from the transistor 500 and isa transistor which is formed similarly to the transistor 602. In otherwords, it is preferable for the transistor 600 to include an oxidesemiconductor layer which is used for a channel formation region. Inaddition, the channel widths of the transistor 602 and the transistor604 are preferably much larger than that of the transistor 600, furtherpreferably three times or more that of the transistor 600, still furtherpreferably five times or more that of the transistor 600.

In FIG. 12C, the transistor 600 is provided over a substrate 616. Thetransistor 600 is covered with an insulating layer, and the insulatinglayer is subjected to planarization treatment using CMP or the like, sothat the gate electrode of the transistor 600 is exposed. Above theexposed gate electrode of the transistor 600, part of the same layer ofsource and drain electrode layers of the transistor 602 and thetransistor 604 are provided, whereby the gate electrode of thetransistor 600 and the fourth wiring 614 are electrically connected withthe same layer (not illustrated in FIG. 12C).

As illustrated in FIG. 12C, parts of the same layer as the gateelectrode of the transistor 600 (parts functioning as the back gates ofthe transistor 602 and the transistor 604) overlap with at leastportions functioning as channel formation regions in semiconductorlayers of the transistor 602 and the transistor 604. The partsfunctioning as the back gates of the transistor 602 and the transistor604 and the semiconductor layers of the transistor 602 and thetransistor 604 are provided so that an insulating layer provided overthe transistor 600 is sandwiched therebetween. This insulating layer isa portion of the insulating layer which has been provided over thetransistor 600 left after the planarization treatment, due to a lack ofthe thickness of the semiconductor layer of the transistor 600. Asdescribed above, the transistors in an upper portion and the back gatesare provided with the insulating layer which is left after theplanarization treatment and interposed therebetween, and the back gatesare formed of part of the same layer as the gate electrode of thetransistor in a lower portion, which are one of features of the memoryelement that is one embodiment of the present invention. In such amanner, the back gates of the transistors in an upper portion are formedof part of the same layer as the gate electrode of the transistor in alower portion, whereby the back gates of the transistors in an upperportion can be provided without an increase in the number ofmanufacturing steps.

Embodiment 7

In this embodiment, an element which is an embodiment of the presentinvention and different from those of Embodiment 1 to Embodiment 6 willbe described. Specifically, a NOR gate which is one of logic gates andcan be manufactured in a manner similar to that of Embodiment 1 will bedescribed with reference to FIGS. 13A to 13C.

A memory element illustrated in FIG. 13A includes a transistor 700, atransistor 702, a transistor 704, and a transistor 706. In FIG. 13A, oneof source and drain electrodes of the transistor 700 is connected to afifth wiring 715 at power supply potential V_(dd). The other of thesource and drain electrodes of the transistor 700 is connected to one ofsource and drain electrodes of the transistor 702. The other of thesource and drain electrodes of the transistor 702 is connected to one ofsource and drain electrodes of the transistor 704, one of source anddrain electrodes of the transistor 706, and a third wiring 713. Theother of the source and drain electrodes of the transistor 704 and theother of the source and drain electrodes of the transistor 706 areconnected to a fourth wiring 714 at ground potential V_(ss). A gateelectrode of the transistor 700 and a gate electrode of the transistor706 are connected to a first wiring 711. A gate electrode of thetransistor 702 and a gate electrode of the transistor 704 are connectedto a second wiring 712. The transistor 704 is provided with a back gateBG1 functioning as another gate electrode, and the transistor 706 isprovided with a back gate BG2 functioning as another gate electrode.

FIG. 13B is a top view illustrating a specific example of a structure ofthe memory element of FIG. 13A. FIG. 13C is a cross-sectional view takenalong X-Y of FIG. 13B.

As illustrated in FIG. 13B, each of the transistor 700 and thetransistor 702 can be the same transistor as the transistor 100 of FIGS.1A to 1C. Each of the transistor 704 and the transistor 706 can be thesame transistor as the transistor 102 of FIGS. 1A to 1C.

In FIG. 13C, the transistor 700 (not illustrated in FIG. 13C) and thetransistor 702 are provided over a substrate 716. The transistor 700 andthe transistor 702 are covered with an insulating layer, and theinsulating layer is subjected to planarization treatment using CMP orthe like, so that the gate electrodes of the transistor 700 and thetransistor 702 are exposed. Over the gate electrodes of the transistor700 and the transistor 702, part of the same layer as source and drainelectrode layers of the transistor 704 and the transistor 706 areprovided, whereby the gate electrode of the transistor 700 and the gateelectrode of the transistor 702 are electrically connected to the firstwiring 711 and the second wiring 712, respectively, with the same layers(not illustrated in FIG. 13C). Note that the transistor 700 and thetransistor 702 here are p-channel transistors but not limited thereto.

As illustrated in FIG. 13C, parts of the gate electrodes of thetransistor 700 (not illustrated in FIG. 13C) and the transistor 702(parts functioning as the back gates of the transistor 704 and thetransistor 706) overlap with at least channel formation regions insemiconductor layers of the transistor 704 and the transistor 706. Theparts functioning as the back gates of the transistor 704 and thetransistor 706 and the semiconductor layers of the transistor 704 andthe transistor 706 are provided so that an insulating layer providedover the transistor 700 and the transistor 702 is sandwichedtherebetween. This insulating layer is a portion of the insulating layerwhich has been provided over the transistor 700 and the transistor 702left after the planarization treatment, due to the a lack of thicknessesof the semiconductor layers of the transistor 700 and the transistor702. As described above, the transistors in an upper portion and theback gates are provided with the insulating layer which is left afterthe planarization treatment and interposed therebetween, and the backgates are formed of parts of the same layer as the gate electrodes ofthe transistors in a lower portion, which are one of features of thememory element that is one embodiment of the present invention. In sucha manner, the back gates of the transistors in an upper portion areformed of parts of the same layer as the gate electrodes of thetransistors in a lower portion, whereby the back gates of thetransistors in an upper portion can be provided without an increase inthe number of manufacturing steps.

Embodiment 8

In this embodiment, an element which is an embodiment of the presentinvention and different from those of Embodiment 1 to Embodiment 7 willbe described. Specifically, a NOR gate which is one of logic gates andcan be manufactured in a manner similar to that of Embodiment 2 will bedescribed with reference to FIGS. 14A to 14C.

A memory element illustrated in FIG. 14A includes a transistor 800, atransistor 802, and a transistor 804. In FIG. 14A, one of source anddrain electrodes of the transistor 800 and one of source and drainelectrodes of the transistor 802 are connected to a fifth wiring 815 atground potential V_(ss). The other of the source and drain electrodes ofthe transistor 800, the other of the source and drain electrodes of thetransistor 802, and one of source and drain electrodes of the transistor804 are connected to a third wiring 813. The other of the source anddrain electrodes of the transistor 804 is connected to a fourth wiring814 of power supply V_(dd). A gate electrode of the transistor 800 isconnected to a first wiring 811. A gate electrode of the transistor 802is connected to a second wiring 812. A gate electrode of the transistor804 is connected to the other of the source and drain electrodes of thetransistor 804. The transistor 800 is provided with a back gate BG1functioning as another gate electrode. The transistor 802 is providedwith a back gate BG2 functioning as another gate electrode. Thetransistor 804 is provided with a back gate BG3 functioning as anothergate electrode.

FIG. 14B is a top view illustrating a specific structure of the memoryelement of FIG. 14A. FIG. 14C is a cross-sectional view taken along X-Yof FIG. 14B.

As illustrated in FIG. 14B, the transistor 800 and the transistor 802can be the same transistors as the transistor 704 and the transistor 706of FIGS. 13A to 13C.

However, the transistor 804 is different from the transistor 700 and thetransistor 702 and is a transistor which is formed similarly to thetransistor 802. In other words, it is preferable for the transistor 804to include an oxide semiconductor layer which is used for a channelformation region. In addition, the channel widths of the transistor 800and the transistor 802 are preferably much larger than that of thetransistor 804, further preferably three times or more that of thetransistor 804, still further preferably five times or more that of thetransistor 804.

In FIG. 14C, the transistor 804 is provided over a substrate 816. Thetransistor 804 is covered with an insulating layer, the insulating layeris subjected to planarization treatment using CMP or the like, so thatthe gate electrode of the transistor 804 is exposed. Above the exposedgate electrode of the transistor 804, part of the same layer as sourceand drain electrode layers of the transistor 800 and the transistor 802is provided, whereby the gate electrode of the transistor 804 and thefourth wiring 814 are electrically connected with the same layer (notillustrated in FIG. 14C).

As illustrated in FIG. 14C, parts of the same layer as the gateelectrode of the transistor 804 (parts functioning as the back gates ofthe transistor 800 and the transistor 802) overlap with at leastportions functioning as channel formation regions in semiconductorlayers of the transistor 800 and the transistor 802. The partsfunctioning as the back gates of the transistor 800 and the transistor802 and the semiconductor layers of the transistor 800 and thetransistor 802 are provided so that an insulating layer provided overthe transistor 804 is sandwiched therebetween. This insulating layer isa portion of the insulating layer which has been provided over thetransistor 804 left after the planarization treatment, due to thethickness of the semiconductor layer of the transistor 804. As describedabove, the transistors in an upper portion and the back gates areprovided with the insulating layer which is left after the planarizationtreatment and interposed therebetween, and the back gates are formed ofpart of the same layer as the gate electrode of the transistor in alower portion, which are one of features of the memory element that isone embodiment of the present invention. In such a manner, the backgates of the transistors in an upper portion are formed of part of thesame layer as the gate electrode of the transistor in a lower portion,whereby the back gates of the transistors in an upper portion can beprovided without an increase in the number of manufacturing steps.

Embodiment 9

In this embodiment, electronic devices which are one embodiment of thepresent invention will be described. In the electronic devices of thisembodiment, at least one of elements described in Embodiment 1 toEmbodiment 8 is mounted. Examples of the electronic devices of thepresent invention include computer, a mobile phone (also referred to asa cellular phone or a mobile phone device), a portable informationterminal (including a portable game machine, an audio reproducingdevice, and the like), a digital camera, a digital video camera,electronic paper, and a television device (also referred to as atelevision or a television receiver).

FIG. 15A illustrates a laptop personal computer including a housing 901,a housing 902, a display portion 903, a keyboard 904, and the like. Theelement described in any of Embodiment 1 to Embodiment 8 is provided inthe housing 901 and the housing 902. The memory described in any ofEmbodiment 1 to Embodiment 8 is mounted on the laptop personal computerillustrated in FIG. 15A, whereby consumed power and the area occupied bythe element can be reduced.

FIG. 15B illustrates a personal digital assistant (PDA) in which a mainbody 911 is provided with a display portion 913, an external interface915, operation buttons 914, and the like. Further, a stylus 912 foroperating the portable information terminal or the like is provided. Theelement described in any of Embodiment 1 to Embodiment 8 is provided inthe main body 911. The memory described in any of Embodiment 1 toEmbodiment 8 is mounted on the PDA illustrated in FIG. 15B, wherebyconsumed power and the area occupied by the element can be reduced.

FIG. 15C illustrates an electronic book reader 920 mounting electronicpaper. The electronic book reader 920 has two housings of a housing 921and a housing 923. The housing 921 and the housing 923 are provided witha display portion 925 and a display portion 927, respectively. Thehousing 921 and the housing 923 are connected by a hinge 937 and can beopened and closed with the hinge 937 as an axis. Further, the housing921 is provided with a power switch 931, operation keys 933, a speaker935, and the like. At least one of the housing 921 and the housing 923is provided with the memory described in any of Embodiment 1 toEmbodiment 8. The memory described in any of Embodiment 1 to Embodiment8 is mounted on the electronic book reader illustrated in FIG. 15C,whereby consumed power and the area occupied by the element can bereduced.

FIG. 15D illustrates a mobile phone including two housings of a housing940 and a housing 941. Further, the housing 940 and the housing 941 in astate where they are developed as illustrated in FIG. 15D can shift bysliding so that one is lapped over the other; therefore, the size of themobile phone can be reduced, which makes the mobile phone suitable forbeing carried. The housing 941 is provided with a display panel 942, aspeaker 943, a microphone 944, an operation key 945, a pointing device946, a camera lens 947, an external connection terminal 948, and thelike. The housing 940 is provided with a solar cell 949 that charges themobile phone, an external memory slot 950, and the like. Note that anantenna is incorporated in the housing 941. At least one of the housing940 and the housing 941 is provided with the element described in any ofEmbodiment 1 to Embodiment 8. The memory described in any of Embodiment1 to Embodiment 8 is mounted on the mobile phone illustrated in FIG.15D, whereby consumed power and the area occupied by the element can bereduced.

FIG. 15E illustrates a digital camera including a main body 961, adisplay portion 967, an eyepiece 963, an operation switch 964, a displayportion 965, a battery 966, and the like. The memory described in any ofEmbodiment 1 to Embodiment 8 is provided in the main body 961. Thememory described in any of Embodiment 1 to Embodiment 8 is mounted onthe digital camera illustrated in FIG. 15E, whereby consumed power andthe area occupied by the element can be reduced.

FIG. 15F is a television device 970 including a housing 971, a displayportion 973, a stand 975, and the like. The television device 970 can beoperated by an operation switch of the housing 971 or a separate remotecontroller 980. The housing 971 and the remote controller 980 areprovided with the memory described in any of Embodiment 1 to Embodiment8. The memory described in any of Embodiment 1 to Embodiment 8 ismounted on the television device illustrated in FIG. 15F, wherebyconsumed power and the area occupied by the element can be reduced.

This application is based on Japanese Patent Application serial no.2010-035435 filed with Japan Patent Office on Feb. 19, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising acircuit, the circuit comprising: a first transistor comprising: a firstsemiconductor layer; a first gate insulating layer over the firstsemiconductor layer; and a first gate electrode over the first gateinsulating layer; an insulating layer over the first semiconductorlayer; and a second transistor comprising: a second gate electrode; asecond gate insulating layer over the second gate electrode, the secondgate insulating layer comprising part of the insulating layer; and asecond semiconductor layer over the second gate insulating layer,wherein the first semiconductor layer includes silicon, wherein thesecond semiconductor layer includes an oxide semiconductor, wherein thesecond gate electrode is formed from a same layer as the first gateelectrode, and wherein the first gate electrode is electricallyconnected to one of a source and a drain of the second transistor. 2.The semiconductor device according to claim 1, wherein the firsttransistor is supported by an SOI substrate.
 3. The semiconductor deviceaccording to claim 1, wherein the first transistor is supported by asilicon substrate.
 4. The semiconductor device according to claim 1,wherein the insulating layer has a leveled upper surface.
 5. Thesemiconductor device according to claim 1, wherein a top surface of thefirst gate electrode is not covered by the insulating layer.
 6. Thesemiconductor device according to claim 1, wherein a distance betweenthe second gate electrode and a channel formation region of the secondsemiconductor layer is equal to a thickness of the first semiconductorlayer.
 7. The semiconductor device according to claim 1, wherein thecircuit is a memory element.
 8. A semiconductor device comprising acircuit, the circuit comprising: a first transistor comprising: a firstsemiconductor layer; a first gate insulating layer over the firstsemiconductor layer; and a first gate electrode over the first gateinsulating layer; an insulating layer over the first semiconductorlayer; a second transistor comprising: a second gate electrode; a secondgate insulating layer over the second gate electrode, the second gateinsulating layer comprising part of the insulating layer; and a secondsemiconductor layer over the second gate insulating layer; and acapacitor comprising: a first electrode formed from a same layer as thefirst semiconductor layer; and a second electrode over the firstelectrode, wherein the insulating layer is between the secondsemiconductor layer and the second gate electrode, wherein the firstsemiconductor layer includes silicon, wherein the second semiconductorlayer includes an oxide semiconductor, wherein the second gate electrodeis formed from a same layer as the first gate electrode, wherein thesecond electrode is electrically connected to the first gate electrode,and wherein the second electrode is electrically connected to one of asource and a drain of the second transistor.
 9. The semiconductor deviceaccording to claim 8, wherein the second electrode is formed from a samelayer as the first gate electrode.
 10. The semiconductor deviceaccording to claim 8, wherein the first transistor is supported by anSOI substrate.
 11. The semiconductor device according to claim 8,wherein the first transistor is supported by a silicon substrate. 12.The semiconductor device according to claim 8, wherein the insulatinglayer has a leveled upper surface.
 13. The semiconductor deviceaccording to claim 8, wherein a top surface of the first gate electrodeis not covered by the insulating layer.
 14. The semiconductor deviceaccording to claim 8, wherein a distance between the second gateelectrode and a channel formation region of the second semiconductorlayer is equal to a thickness of the first semiconductor layer.
 15. Thesemiconductor device according to claim 8, wherein the circuit is amemory element.
 16. A semiconductor device comprising a circuit, thecircuit comprising: a first transistor comprising: a first semiconductorlayer; a first gate insulating layer over the first semiconductor layer;and a first gate electrode over the first gate insulating layer; aninsulating layer over the first semiconductor layer; and a secondtransistor comprising: a second gate electrode; a second gate insulatinglayer over the second gate electrode, the second gate insulating layercomprising part of the insulating layer; and a second semiconductorlayer over the second gate insulating layer, wherein the firstsemiconductor layer includes silicon, wherein the second semiconductorlayer includes an oxide semiconductor, wherein the second gate electrodeis formed from a same layer as the first gate electrode, wherein thecircuit is an inverter element, wherein the first gate electrode isformed from a same layer as the second gate electrode, and wherein oneof a source and a drain of the first transistor is electricallyconnected to one of a source and a drain of the second transistor. 17.The semiconductor device according to claim 16, wherein the firsttransistor is supported by an SOI substrate.
 18. The semiconductordevice according to claim 16, wherein the first transistor is supportedby a silicon substrate.
 19. The semiconductor device according to claim16, wherein the insulating layer has a leveled upper surface.
 20. Thesemiconductor device according to claim 16, wherein a top surface of thefirst gate electrode is not covered by the insulating layer.
 21. Thesemiconductor device according to claim 16, wherein a distance betweenthe second gate electrode and a channel formation region of the secondsemiconductor layer is equal to a thickness of the first semiconductorlayer.